The present technology relates to a technical field concerning to a memory device including a memory element configured to store information on the basis of changes in resistance and a method for accessing the memory device.
With the dramatic development of various types of information devices, including mobile terminals and high-capacity servers, elements of these information devices, such as memory devices or logic circuits, have to have higher performance, such as higher integration, higher speed, or lower power consumption.
Among others, non-volatile semiconductor memory has significantly developed, and flash memory as high-capacity file memory has become widespread with momentum to expel hard disk drives.
Further, non-volatile semiconductor memory is being developed to replace NOR flash memory, DRAM, and the like which are currently commonly used in code storage and working memory applications. Examples of such non-volatile semiconductor memory include magnetoresistive random access memory (MRAM), resistive random access memory (ReRAM), and phase-change random access memory (PCRAM). These types of memory each include a memory element configured to store data on the basis of changes in resistance and commonly called resistive memory.
Of these types of non-volatile memory, MRAM stores data on the basis of the magnetization direction of the magnetic body and thus allow the data to be rewritten fast and approximately an infinite number of times (1015 or more). Owing to the fast operation and reliability thereof, MRAM is expected to be developed into code storage or working memory.
As described in U.S. Pat. No. 5,695,864 and Japanese Unexamined Patent Application Publication No. 2003-17782, methods for writing to MRAM include magnetic-field writing, where the magnetization is inverted using a current magnetic field generated by the lines, and spin-injection writing, where the magnetization is inverted using spin transfer torque.
Compared to magnetic-field writing, spin-injection writing obviates the necessity for the lines for generating a current magnetic field and therefore advantageously simplifies the cell structure. Hereafter, spin-injection-writing MRAM will be simply referred to as MRAM.
An MRAM cell includes a magnetic tunnel junction (MTJ).
An MTJ is a tunnel junction formed by disposing a tunnel barrier layer between a magnetic layer whose magnetization direction is fixed to a certain direction (hereafter referred to as the magnetization-fixed layer) and a magnetic layer whose magnetization direction is not fixed (hereafter referred to as the storage layer).
Information “0” or “1” is read from the memory cell by using the so-called tunnel magneto-resistance effect, in which the resistance of the MTJ varies with the relative angle between the magnetization directions of the magnetization-fixed layer and storage layer.
On the other hand, “0” or “1” is written to the memory cell by using the fact that when spin-polarized electrons which have passed through the magnetization-fixed layer enter the storage layer, they give a torque to this magnetic layer. When a current higher than or equal to a certain threshold passes through the storage layer, the magnetization direction of the storage layer is inverted.